Controlling heat sources based on representative temperatures

ABSTRACT

In an example, a method includes measuring a temperature of a plurality of regions of a layer of build material in an additive manufacturing apparatus to provide initial temperature values. For each of a plurality of regions which comprise build material which is intended to fuse, an average temperature value of a plurality of neighbouring regions may be determined and the initial temperature values may be replaced with the average temperature value. Based on the replacement temperature values, a representative temperature of an area of the layer of build material may be determined and a heat source may be controlled based on the representative temperature.

BACKGROUND

Additive manufacturing techniques may generate a three-dimensionalobject through the solidification of a build material, for example on alayer-by-layer basis. In examples of such techniques, build material maybe supplied in a layer-wise manner and the solidification method mayinclude heating the layers of build material to cause melting inselected regions. In other techniques, chemical solidification and/orbinding methods may be used.

BRIEF DESCRIPTION OF DRAWINGS

Non-limiting examples will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is an example method of controlling a heat source in additivemanufacturing;

FIG. 2A-2F are examples of pixels in heat maps;

FIG. 3 is an example of a method of determining an average value;

FIG. 4 is an example of a method for determining a representativetemperature in additive manufacturing;

FIG. 5 is an example of a heat map;

FIG. 6 is an example of a chart for determining a representativetemperature in additive manufacturing;

FIG. 7 is an example of an additive manufacturing apparatus;

FIG. 8 is another example of an additive manufacturing apparatus; and

FIG. 9 is an example machine readable medium associated with aprocessor.

DETAILED DESCRIPTION

Additive manufacturing techniques may generate a three-dimensionalobject through the solidification of a build material. In some examples,the build material is a powder-like granular material, which may forexample be a plastic, ceramic or metal powder and the properties ofgenerated objects may depend on the type of build material and the typeof solidification mechanism used. Build material may be deposited, forexample on a print bed and processed layer by layer, for example withina fabrication chamber.

In some examples, selective solidification is achieved throughdirectional application of energy, for example using a laser or electronbeam which results in solidification of build material where thedirectional energy is applied. In other examples, at least one printagent may be selectively applied to the build material, and may beliquid when applied.

Some 3D printing technology works by generating layers of a giventhickness, one on top of another. Build material may be deposited, forexample, on a print bed, and processed layer by layer, for examplewithin a fabrication chamber or “build volume” of the printer. The buildmaterial may be a powder-like granular material, which may for examplebe a plastic, ceramic or metal powder. According to one example, asuitable build material may be PA12 build material commercially known asV1R10A “HP PA12” available from HP Inc.

In some examples, at least one print agent may be selectively applied tothe build material, and may be liquid when applied. For example, afusing agent (also termed a “coalescence agent” or “coalescing agent”)may be selectively distributed onto portions of a layer of buildmaterial in a pattern derived from data representing a slice of athree-dimensional object to be generated (which may for example begenerated from structural design data). The fusing agent may have acomposition which absorbs energy such that, when energy (for example,heat) is applied to the layer, the build material coalesces andsolidifies to form a slice of the three-dimensional object in accordancewith the pattern.

According to one example, a suitable fusing agent may be an ink-typeformulation comprising carbon black, such as, for example, the fusingagent formulation commercially known as V1Q60Q “HP fusing agent”available from HP Inc. In some examples such a fusing agent mayadditionally comprise an infra-red light absorber. In some examples sucha fusing agent may additionally comprise a near infra-red lightabsorber. In some examples such a fusing agent may additionally comprisea visible light absorber. In some examples such a fusing agent mayadditionally comprise a UV light absorber. Examples of print agentscomprising visible light enhancers are dye based colored ink and pigmentbased colored ink, such as inks commercially known as CE039A and CE042Aavailable from HP Inc.

In other examples, coalescence may be achieved in some other manner.

In some examples, a detailing agent may also be used (also termed a“coalescence modifier agent” or “coalescing modifier agent”), which mayhave a cooling effect. In some examples, the detailing agent may be usednear edge surfaces of an object being printed. According to one example,a suitable detailing agent may be a formulation commercially known asV1Q61A “HP detailing agent” available from HP Inc.

A coloring agent, for example comprising a dye or colorant, may in someexamples be used as a fusing agent or a coalescence modifier agent,and/or as a print agent to provide a particular color for the object.Print agents may control or influence other physical or appearanceproperties, such as strength, resilience, conductivity, transparency,surface texture or the like.

In some examples of additive manufacturing, heat sources (for example,infrared emitters such as a heat lamp, which may comprise staticallymounted heat sources directed at a print bed) are controlled to maintainan appropriate temperature range during object generation. In someexamples, a feedback loop is used, and one or more heat sources arecontrolled according to a measured temperature. In some examples, alayer of build material is divided into a plurality of print bed zones,each associated with a heat source. An average or representativetemperature of the build material in each zone may be determined, andthe heat source in turn controlled according to that temperature.

In some examples, the reference temperature to be used for a feedbackloop is intended to be the temperature of build material in the absenceof print agent, which will be referred to herein as ‘blank’ buildmaterial.

However, during an object generation process, portions of a layer ofbuild material may be treated with a print agent, for example any of theprint agents described above. For example, fusing agent will tend toincrease the temperature in a region and, in particular where theresolution with which the temperature is within that region can bemeasured is relatively low, this can make it difficult to correctlydetermine the temperature of blank build material, as some temperaturemeasurements may include both treated and blank build material, or heatmay be transferred between treated and blank build material, and this inturn can result in incorrect or sub-optimal control of the heat sources.

FIG. 1 is an example of a method of additive manufacturing, which may bea layer by layer object generation process, otherwise known as 3Dprinting. In block 102, the temperature of each of a plurality ofregions of a layer of build material in an additive manufacturingapparatus is measured to provide initial temperature values. In someexamples, a ‘heat map’ of the layer of build material may be acquired,for example using one or more thermal cameras. The heat map may be madeup of an array of pixels, each corresponding to a region of the layer ofbuild material, and the temperature of each of such pixels/regions maybe monitored. For example, the array may be on the order of 22-40 pixelssquare, in some examples being 32 pixels square.

Block 104 comprises determining, for each of a plurality of regionswhich comprise build material which is intended to fuse, an averagetemperature value of a plurality of neighbouring regions and block 106comprises replacing the initial temperature values with the averagetemperature values. For example, a pixel which comprises a thermal imageof a region of the build material which includes at least some buildmaterial which is intended to fuse, or which is closer there to, mayhave an initial temperature value. This temperature value may, ingeneral, be higher than a temperature value of blank or untreated buildmaterial. In this example, such values are replaced by an averagetemperature value determined by averaging neighbouring regions. Theneighbouring regions may comprise regions in which the initial measuredtemperature is a temperature associated with blank build material and/orpreviously replaced initial temperature values. In some examples, theinitial values of a plurality of regions which comprise build materialwhich is intended to fuse may first be replaced by a null value beforebeing replaced with an average temperature value.

In some examples, the regions which comprise build material which isintended to fuse are determined, at least in part, by reference tocontrol data used to instruct the distribution of print agents. Suchcontrol data may be generated based on object model data representing atleast a portion of an object to be generated by an additivemanufacturing apparatus by fusing build material. The object model datamay for example comprise a Computer Aided Design (CAD) model, and/or mayfor example be a STereoLithographic (STL) data file.

In some examples, these regions may be identified by temperature. Forexample regions in which pixels which are above a threshold temperaturemay be identified as relating to build material which is intended tofuse.

Block 108 comprises determining, based on the replacement temperaturevalues, a representative temperature of an area of the layer of buildmaterial. For example, this may comprise selecting an area of the layerof build material corresponding to a particular heat source (a ‘printbed zone’). An average temperature, which may include an average ofinitial temperatures and/or of at least some replacement temperaturesdetermined in blocks 104 and 106 may be determined as a representativetemperature for this area. In other examples, the area may comprise theprint bed as a whole.

Block 110 comprises controlling a heat source based on therepresentative temperature. This may for example comprise controlling apower level of a heat source heating the layer of build material.Controlling the power level may comprise controlling the average powerlevel over time, for example using pulse width modulation control, whichsets the percentage of time for which a heating element is emittinglight. In other examples, the power level of a constant heat output maybe controlled.

In some examples, the heat sources may be, for example heat lamps, suchas infrared heat lamps. In some examples, there may be an array of heatsources overlying a print bed in an additive manufacturing apparatus. Insome examples, there may be on the order of 10, 20, or 50 heat sources.Controlling the heat source in block 110 may be carried out as a‘closed-loop’ control process.

By replacing temperatures associated with regions of the build materialwhich are intended to fuse (i.e. those which have fusing agent appliedthereto) with average temperatures, a temperature which is morerepresentative of the blank build material temperature may bedetermined. When compared to, for example, simply replacing a ‘hot’pixel with a temperature value from a neighbouring pixel, the method ofFIG. 1 results in improved accuracy.

FIG. 2A shows an example of a heat map 200 of a layer of build materialduring object generation in which centigrade temperature measurementsare given as whole numbers (although in practice, temperatures may berecorded to a higher level of accuracy). In a portion 202 of the layer(which comprises a plurality of pixels/regions of the build materialwhich is intended to fuse), solidification of the build material istaking place and the temperature of the pixels therein is generallyhigher. In addition, it may be noted that in the edge regions 204 a-d,the temperature values are generally lower. This is because heat is lostto the environment surrounding the fabrication chamber. It may be notedthat the edge region is larger in the corners (or tapers with distancefrom the corner). This is, at least in part, an artefact of the thermalcamera used to measure the temperatures in this example, which is usedin association with a fisheye lens. Therefore, the pixels at the cornerscorrespond to a different size of region of the build material thanthose at the centre. This may not be the case in all examples, forexample if an array of temperature sensors which was of a similar sizeto the print bed was used and therefore such regions may not be presentor may differ in appearance in other examples.

FIG. 2B, shows a version of this in which the data relating to theportion 202 which is intended to fuse and the edge regions 204 has beenremoved. In some examples, this data may be replaced with a null valueof 0 (zero). The remaining temperature values are indicative of thetemperature of the blank build material in the layer.

FIGS. 2C and 2D show the result of replacing values with an averagevalue in a first replacement operation. As is shown in FIG. 2D, in thisexample an average value for a pixel 206 may be determined from fourneighbouring pixels 208 a-d (which in this example are shown to twodecimal places), where neighbouring pixels 208 are selected from a firstand second side of the pixel 206 to be overwritten. In this example, thedata is replaced in a ‘sweep’ which starts from the top left-hand of theheat map and extends through to the bottom right of the heat map. Inthis example, the sweep starts with the first null pixel 206 which has aset of non-null neighbour pixels 208. While four neighbour pixels 208have been used for determining the average in this example, more orfewer pixels may be used in other examples. In addition, while theneighbour pixels are direct neighbours in this example (i.e. they adjointhe pixel whose value is to be replaced), in other examples, theneighbour pixels may be from the vicinity of the pixel whose value is tobe replaced, but not directed adjoining that pixel.

Therefore, selecting neighbouring pixels which are to the left and abovethe pixel to be overwritten sweeps data through the heat map 200. Insome examples, there may be a second sweep through the data, for examplestarting from the bottom right-hand corner and ending in the topleft-hand corner. In the second sweep, an average may be based on pixelswhich are to the right and below the pixel to be overwritten, as isshown in FIG. 2E (in which T_(ave) is the average of T₁, T₂, T₃ and T₄),and FIG. 2F. This may propagate values into the top and/or left-handcorners. In this example, all pixels which are shown as null value or‘0’ pixels in FIG. 2B are replaced (meaning that some null pixels may beoverwritten twice: first with an initial replacement average value andthen with a second replacement average value).

Thus, FIG. 2F shows an example of a heat map with the temperature valuesof a portion 202 which comprises build material which is intended tofuse and the temperature values of regions 204 which are in a boundaryzone of the layer have been replaced with an average temperature basedon neighbouring regions.

FIG. 3 is an example of a method for carrying out blocks 104 and 106 ofFIG. 1.

Block 302 comprises, determining, for a plurality of regions, a firstreplacement temperature value comprising a weighted average value ofneighbouring regions on a first and second side of each region. Forexample, this may comprise the left and top side of the pixel, asdescribed above in relation to FIGS. 2C and 2D. Therefore, this maycomprise carrying out a first sweep e.g. top left to bottom right asdescribed above. Block 304 comprises determining, for a plurality ofregions, a second replacement temperature value comprising a weightedaverage value of neighbouring regions on a third and fourth side region.This may for example comprise the right and under side of the pixel, asdescribed above in relation to FIGS. 2E and 2F. In some examples, thismay comprise carrying out a second sweep, e.g. bottom right top left asdescribed above. More generally, the first and second sides may beadjoining orthogonal sides, and the third and fourth sides may beadjoining orthogonal sides.

In this example, in both the first and second sweep, the temperaturevalues of pixels/regions which are not initially representative of thetemperature of blank build material may be replaced, whereas thetemperature values of pixels/regions which are representative of thetemperature of blank build material may remain as initially measured.However in other examples, additional smoothing may be carried out,and/or a first replacement average value may not be overwritten by asecond replacement average value.

In the example of FIG. 3, the average values are weighted values. Forexample, the weighting may be indicative of whether the temperaturevalue of the neighbouring region is an initial value or a replacementvalue. Further, in some examples, the weighting may indicate the numberof initial values which we used to determine the replacement value.Thus, for the sake of example, if a value contributing to the average isan initial value, this may have a weighting of 5. If the value is areplacement value which was determined using three initial values andone replacement value, this may have a weighting of 4. If the value is areplacement value which was determined using two initial values and tworeplacement values, it may have a weighting of 3. If the value is areplacement value which was determined using one initial value and threereplacement values, it may have a weighting of 2. If the value is areplacement value which was determined using four replacement values, itmay have a weighting of 1.

In other examples a more complete history of the weight of initialvalues remaining in a replacement value may be used to determine itsweighting in an average.

This method may be carried out for regions of the build material whichare intended to fuse and/or for at least one region which are within theboundary zone of the layer. In such examples, a representativetemperature of an area of the layer of build material may be determinedbased on the replacement temperature values of the regions whichcorrespond to portions of the layer which are intended to fuse and thosewhich are in the boundary zone.

FIG. 4 is another example of a method of additive manufacturingcomprising, in block 402, measuring the temperature of a plurality ofregions of the layer of build material in an additive manufacturingapparatus to provide initial temperature values. The operation may becarried out as described above in relation to block 102.

Block 404 comprises determining the size of a continuous area of thelayer of build material which comprises build material which is intendedto fuse. This may be determined based on control data, object modeldata, or thermal data such as measured temperature, as described abovein relation to block 404. In some examples, there may be several regionswhich are intended to fuse and each of these regions may be consideredindividually.

Block 406 comprises comparing the size to a threshold size. In the eventthat the size is below the threshold size, the method proceeds withblock 104 of FIG. 1. However, in the event that the size is above thethreshold size, the method continues with block 408 which comprisesusing a linear regression and a predicted heat of each region based ondata indicating the portions of the layer which are intended to fuse toestimate the representative temperature.

Therefore, in this example, there are two methods which may be used toreplace data to determine a representative temperature. The method ofFIG. 1 may be used for continuous regions up to a threshold size and themethod of block 408 may be used for regions above this size. Thethreshold size may be determined for each of a plurality of print bedzones, the zones being associated with different heat sources.

FIG. 5 shows an example of a heat map of a zone of a layer of buildmaterial associated with a particular heat source in which a largeproportion (the white portion) is intended to fuse. The zone may be oneof, for example twelve zones of the layer.

In some examples, a predicted heat of each region may be determinedbased on an amount of print agent to be applied thereto and, in someexamples, the predicted temperature of surrounding regions. Whenpredicting temperatures, any optical distortion, for example due tofisheye lens(es) applied to the thermal camera, as is shown in FIG. 5,or the like, may be taken into account.

This can be used to determine a ‘grey level’ characterising the expectedtemperature of each pixel. In some examples, untreated powder may beassociated with a grey level value of 0. However, as detailed above, insome examples detailing agent may be used to cool powder. Therefore, insome examples, the grey level associated with such detailing agents mayfor example be in the region of 0 to 20, the white powder may beassociated with a grey level of around 20 to 25 (in some examples 23)and pixels associated with the application of fusing agent may beassociated with higher grey level values. The grey levels for each pixelmay then be plotted against the measured temperature value for thatpixel. In some examples, an average of the measured temperatures for allpixels with a particular assigned grey level may be determined.

For example, in a particular zone it may be expected that eight pixelsare associated with the temperature of blank build material (in thisexample grey level equal to 0), six pixels may be associated with a greylevel of 9 (for example, this may represent pixels with some thermalcontribution from adjacent pixels to which fusing agent has beenapplied), two pixels may be associated with a grey level of 28, twopixels may be associated with a grey level of 61 and one pixel may beassociated with a grey level of 102 (i.e. a reasonably large amount offusing agent may be applied to the region of the build material capturedin that pixel). In this example, the average measured temperature of thepixels with a grey level of 0 is 154.50° C. The average measuredtemperature value of the pixels with a grey level of 9 is 156.00° C. Theaverage measured temperature of the pixels with a grey level of 28 is155.50° C. The measured temperature of the pixel with a grey level of 61is 156.00° C. and the measured temperature of the pixel with a greylevel of 102 is 157.00° C.

FIG. 6 shows a plot of these grey levels against the averages of themeasured temperatures. The zero crossing point of a ‘best fit’ lineprovides the representative temperature in this example. Therefore, thismethod uses linear regression to determine the representativetemperature.

In some examples, the linear regression may be carried out for aplurality of zones, each zone being associated with a different heatsource. In one example, a linear regression may be carried out for eachof a plurality of zones separately.

FIG. 7 is an example of an additive manufacturing apparatus 700comprising processing circuitry 702, the processing circuitry 702comprises a temperature determination module 704 and a controller 706.

The temperature determination module 704 is, in use of the apparatus, todetermine a representative temperature of the layer of build material byreplacing measured temperature values of a pixel of a heat map of thelayer which comprises build material which is intended to fuse withaverage temperatures of neighbouring pixels. For example, this maycomprise carrying out the methods of FIG. 1 and FIG. 2 described above.

The controller 706 is to control at least one heat source based on therepresentative temperature value of the layer of build material.

FIG. 8 shows an example of an additive manufacturing apparatus 800comprising the processing circuitry 702 including the temperaturedetermination module 704 and the controller 706 of FIG. 7. In addition,the additive manufacturing apparatus 800 comprises a plurality of heatsources 802 a-d and a print bed 804 (although it will be understood thatthe print bed 804 and/or the heat sources 802 a-d may be suppliedseparately from other additive manufacturing apparatus components).

The heat sources 802 are arranged in an array, each heat source 802being associated with a zone of the print bed 804. In this example, thetemperature determination module 704 is to determine a plurality ofrepresentative temperature is a different zones of a layer of buildmaterial and the controller 706 is to control a plurality of heatsources based on the representative temperature value for an associatedzone.

In some examples, the additive manufacturing apparatus 700, 800 maycomprise or otherwise be provided with or operate in association withtemperature sensing apparatus. Such temperature sensing apparatus mayfor example comprise a thermal imaging camera to obtain a thermalimage/heat map of the print bed, wherein the thermal image/heat mapcomprises a plurality of pixels, each pixel having an associatedmeasured temperature. In other examples, temperature sensing apparatusmay comprise a thermal imaging sensor array, or some other thermalsensing apparatus, and may be used to determine one or more temperatures(which may be pixels of a heat map).

The additive manufacturing apparatus 700, 800 may generate objects in alayer-wise manner by selectively solidifying portions of layers of buildmaterials. The selective solidification may in some examples be achievedby selectively applying print agents, for example through use of‘inkjet’ liquid distribution technologies, and applying energy, forexample heat, to each layer. The additive manufacturing apparatus 700,800 may comprise additional components not shown herein, for example afabrication chamber, at least one print head for distributing printagents, a build material distribution system for providing layers ofbuild material and the like.

The additive manufacturing apparatus 700, 800 may, in some examples,carry out at least one of the blocks of FIG. 1, 3 or 4.

FIG. 9 shows an example of a tangible machine readable medium 900 inassociation with a processor 902. The machine readable medium 900 storesinstructions 904 which, when executed by the processor 902 cause theprocessor to carry out actions.

In this example, the instructions 904 comprise instructions 906 toidentify, from a heat map of the layer of build material in an additivemanufacturing apparatus comprising a plurality of pixels, the pixelswhich are directly indicative of a temperature of the layer in theabsence of applied print agent, i.e. the blank pixels. The instructions904 further comprise instructions 908 to cause the processor 902 todetermine, for a plurality of pixels which are not directly indicativeof a temperature of the layer in the absence of applied print agent(e.g. treated and/or edge pixels), an average temperature value of aplurality of neighbouring regions. The instructions 904 further compriseinstructions 910 to cause the processor 902 to determine, based on thedirectly indicative pixels and the determined average temperaturevalues, a representative temperature of an area of the layer of buildmaterial. The instructions 904 further comprise instructions 912 tocause the processor 902 to control a heat source based on therepresentative temperature.

In some examples, the instructions 906 may comprise instructions todetermine the pixels based on data modelling the content of afabrication chamber. In other examples, the blank pixels may beidentified based on control data (for example specifying where printagents have been/are to be placed), and/or measured temperature values,for example based on threshold values.

In some examples, the instructions 908 to determine an averagetemperature value comprise, for at least one pixel which is not directlyindicative of a temperature of the layer in the absence of applied printagent, instructions to determine a first average temperature value of afirst plurality of neighbouring regions, and a second averagetemperature value of a second plurality of neighbouring regions.

In some examples, the instructions 904 may comprise instructions tocause the processor 902 to determine the size of a continuous set ofpixels which are not directly indicative of a temperature of the layerin the absence of applied print agent, and when the area is below athreshold size, determining the average temperatures, and, when the areais at least a threshold size, using a linear regression and a predictedheat of each region to determine a representative temperature.

Examples in the present disclosure can be provided as methods, systemsor machine readable instructions, such as any combination of software,hardware, firmware or the like. Such machine readable instructions maybe included on a computer readable storage medium (including but notlimited to disc storage, CD-ROM, optical storage, etc.) having computerreadable program codes therein or thereon.

The present disclosure is described with reference to flow charts andblock diagrams of the method, devices and systems according to examplesof the present disclosure. Although the flow diagrams described aboveshow a specific order of execution, the order of execution may differfrom that which is depicted. Blocks described in relation to one flowchart may be combined with those of another flow chart. It shall beunderstood that at least some flows and/or blocks in the flow chartsand/or block diagrams, as well as combinations of the flows and/ordiagrams in the flow charts and/or block diagrams can be realized bymachine readable instructions.

The machine readable instructions may, for example, be executed by ageneral purpose computer, a special purpose computer, an embeddedprocessor or processors of other programmable data processing devices torealize the functions described in the description and diagrams. Inparticular, a processor or processing circuitry may execute the machinereadable instructions. Thus functional modules of the apparatus (such asthe temperature determination module 704 and the controller 706) may beimplemented by a processor executing machine readable instructionsstored in a memory, or a processor operating in accordance withinstructions embedded in logic circuitry. The term ‘processor’ is to beinterpreted broadly to include a CPU, processing unit, ASIC, logic unit,or programmable gate array etc. The methods and functional modules mayall be performed by a single processor or divided amongst severalprocessors.

Such machine readable instructions may also be stored in a computerreadable storage that can guide the computer or other programmable dataprocessing devices to operate in a specific mode.

Machine readable instructions may also be loaded onto a computer orother programmable data processing devices, so that the computer orother programmable data processing devices perform a series ofoperations to produce computer-implemented processing, thus theinstructions executed on the computer or other programmable devicesrealize functions specified by flow(s) in the flow charts and/orblock(s) in the block diagrams.

Further, the teachings herein may be implemented in the form of acomputer software product, the computer software product being stored ina storage medium and comprising a plurality of instructions for making acomputer device implement the methods recited in the examples of thepresent disclosure.

While the method, apparatus and related aspects have been described withreference to certain examples, various modifications, changes,omissions, and substitutions can be made without departing from thespirit of the present disclosure. It is intended, therefore, that themethod, apparatus and related aspects be limited by the scope of thefollowing claims and their equivalents. It should be noted that theabove-mentioned examples illustrate rather than limit what is describedherein, and that those skilled in the art will be able to design manyalternative implementations without departing from the scope of theappended claims. Features described in relation to one example may becombined with features of another example.

The word “comprising” does not exclude the presence of elements otherthan those listed in a claim, “a” or “an” does not exclude a plurality,and a single processor or other unit may fulfil the functions of severalunits recited in the claims.

The features of any dependent claim may be combined with the features ofany of the independent claims or other dependent claims, in anycombination.

1. Additive manufacturing apparatus comprising processing circuitry, theprocessing circuitry comprising: a temperature determination module todetermine a representative temperature value for a layer of buildmaterial by replacing measured temperature values of pixels from aplurality of regions of a heat map of the layer which comprise buildmaterial which is intended to fuse with average temperatures ofneighboring pixels; and a controller, to control a heat source based onthe representative temperature value for a layer of build material. 2.Additive manufacturing apparatus of claim 1, in which the temperaturedetermination module is to determine a plurality of representativetemperatures for different zones of the layer, and the controller is tocontrol each of a plurality of heat sources based on the representativetemperature value for a zone associated with that heat source. 3.Additive manufacturing apparatus of claim 2, further comprising aplurality of heat sources.
 4. A machine readable medium comprisinginstructions which, when executed by a processor, cause the processorto: identify, from a heat map of a layer of build material in anadditive manufacturing apparatus comprising a plurality of pixels,pixels which are directly indicative of a temperature of the layer inthe absence of applied print agent; for a plurality of pixels indicativeof a temperature of a plurality of regions of the layer to be fused,determine an average temperature value of a plurality of neighbouringregions; determine, based on the pixels which are directly indicative ofa temperature of the layer in the absence of applied print agent and thedetermined average temperature values, a representative temperature ofan area of the layer of build material; and control a heat source basedon the representative temperature.
 5. A machine readable medium of claim4, wherein the instructions to identify the pixels which are directlyindicative of a temperature of the layer in the absence of applied printagent comprise instructions to identify the pixels based on datamodelling a content of a fabrication chamber.
 6. A machine readablemedium of claim 4, wherein the instructions to determine an averagetemperature value comprise, for at least one pixel which is not directlyindicative of a temperature of the layer in the absence of applied printagent, instructions to determine a first average temperature value of afirst plurality of neighbouring regions and a second average temperaturevalue of a second plurality of neighbouring regions.
 7. A machinereadable medium of claim 4, further comprising instructions to determinea size of a continuous set of pixels which are not directly indicativeof a temperature of the layer in the absence of applied print agent; andwhen the area is below a threshold size, to determine the averagetemperature values, and, when the area is at least a threshold size, touse a linear regression and a predicted heat of each region to determinea representative temperature.
 8. An additive manufacturing apparatuscomprising: An array of temperature sensors for measuring a temperatureof a plurality of regions of a layer of build material in an additivemanufacturing apparatus to provide initial temperature values;processing circuitry comprising a temperature determination module fordetermining, for each of a plurality of regions which comprise buildmaterial which is intended to fuse, an average temperature value of aplurality of neighboring regions, replacing the initial temperaturevalues with the average temperature values, and determining, based onthe replacement temperature values, a representative temperature of anarea of the layer of build material; and a controller for controlling aheat source based on the representative temperature.
 9. The additivemanufacturing apparatus of claim 8, wherein the temperaturedetermination module is programmed to determine the representativetemperature using: a first iteration, for a plurality of regions,determining a first replacement temperature value comprising an averagevalue of neighboring regions on a first and second side of a region; anda second iteration, for a plurality of regions, determining a secondreplacement temperature value comprising an average value of neighboringregions on a third and fourth side of a region, wherein therepresentative temperature is based on the second replacementtemperature values.
 10. The additive manufacturing apparatus of claim 8,wherein the temperature determination module is further programmed for:determining an indication of a size of a continuous area of the layer ofbuild material which comprises build material which is intended to fuse,and determining the replacement temperature values for the area when itis determined that the continuous area is below a threshold size. 11.The additive manufacturing apparatus of claim 10, further comprising,when the temperature determination module determines that the continuousarea is above a threshold size, the temperature determination module isprogrammed to use a linear regression and a predicted heat of eachregion based on data indicating portions of the layer which are intendedto fuse to estimate the representative temperature.
 12. The additivemanufacturing apparatus of claim 8, wherein the average temperaturesvalue is a weighted average, wherein at least one weighting isindicative of whether the temperature value of a neighboring region isan initial value or a replacement value.
 13. The additive manufacturingapparatus of claim 12, wherein the weighting is indicative of how manyinitial values were used in determining a replacement value of thatregion.
 14. The additive manufacturing apparatus of claim 8, wherein thetemperature determination module is further programmed to: determine,for each of a plurality of regions which are in a boundary zone of thelayer, an average temperature value of a plurality of neighboringregions; and replace the initial temperature values with the averagetemperature values, wherein determining, based on the replacementtemperature values, a representative temperature of an area of the layerof build material comprises determining the temperature based on thereplacement temperature values of the regions in the boundary zone. 15.The additive manufacturing apparatus of claim 8, wherein: thetemperature determination module is programmed to determine arepresentative temperature for each of a plurality of print bed zones;and the controller is programmed to control a plurality of heat sourcesbased on the representative temperature of an associated print bed zone.16. The additive manufacturing apparatus of claim 8, wherein thetemperature determination module is programmed to determine each pixelof a thermal image of the layer of build material is used as a regionfor which the average temperature value of a plurality of neighboringregions, an initial temperature value for a number of the pixels beingreplaced with such an average temperature value.
 17. The additivemanufacturing apparatus of claim 16, f wherein the temperaturedetermination module is programmed to determine an average temperaturevalue to replace an initial temperature value for each pixel in a sweeppattern from a first corner of the thermal image to an opposite cornerof the thermal image.
 18. The additive manufacturing apparatus of claim8, wherein each area for which a representative temperature isdetermined comprises a plurality of the regions for which a replacementtemperature is determined, each area corresponding to a different heatsource among a plurality of heat sources.
 19. The additive manufacturingapparatus of claim 8, wherein temperature determination module isfurther programmed to determine each of the plurality of regions whichcomprise build material which is intended to fuse using control datathat instructs distribution of print agents.
 20. The additivemanufacturing apparatus of claim 19, wherein temperature determinationmodule is further programmed to perform a second iteration ofdetermining an average temperature value to replace a currenttemperature value for each pixel in a second sweep pattern from theopposite corner of the thermal image back to the first corner of thethermal image.